Generally in a liquefied gas tank for storage of liquefied gas such as liquefied natural gas (LNG) or liquefied petroleum gas (LPG), open inspection is sometime conducted to check influences due to aged deterioration.
Such open inspection is conducted by a worker or workers who enter into the liquefied gas tank. Thus, before the open inspection, preliminary works are used to completely discharge liquefied gas as content in the liquefied gas tank and replace residual gas in the tank with air to allow the workers to enter into the tank.
Commonly, upon opening the liquefied gas tank, residual stored liquid in the tank is reduced as much as possible into minimization; the stored liquid in the tank is vaporized by use of heat input from ambient air; and heating is further conducted to discharge the vaporized gas from the tank. Such residual liquid discharge method is called as natural heat input type.
For example, Patent Literatures 1 and 2 show general state of art pertinent to a method of discharging residual liquid in a liquefied gas tank.